This invention relates to the production of coated abrasives and specifically to the curing of resin coats by which an abrasive grit is adhered to a backing material in the production of coated abrasives.
In a typical coated abrasive manufacturing process a backing material, which has frequently been treated with a filler coating to make it a better substrate for the application of a binder solution, is coated with a binder resin formulation called a maker coat. Abrasive particles are applied to the coated backing which is then treated to cure the maker coat at least to the extent necessary to hold the particles firmly in place. The coated product thereafter is often given a further binder coat, usually called a size coat, that improves the strength of the bond holding the abrasive particles to the backing. This size coat can also act as a vehicle for the application of a grinding aid or other additive to modify the grinding performance of the coated abrasive. The size coat is then cured.
The resins most frequently used as vehicles for maker and size coats have typically been based on modified or unmodified phenolic resins. While other binders have been suggested, they are subject to problems of control, expense and environmental impact such that they are not universally attractive.
While these alternative binders have their problems, there are also significant problems arising from the use of phenolic resins in such applications.
The first problem arises from the fact that they are used in the form of aqueous dispersions containing about 30-40% of water which must be removed before cure can occur. This drying process, if performed too quickly or at too high a temperature, can result in blisters and bubbles being formed in the bond material which severely reduce the bond strength. Thus typically the drying process is protracted and costly but must be essentially complete before curing can begin.
The phenolic bonding resins when first applied, are lightly or not at all cross-linked. During cure they are increasingly cross-linked and become more and more rigid as a result. The cure process is advanced to different degrees at the various stages of the manufacture of a coated abrasive. The initial cure of the maker coat is advanced only to the extent necessary to ensure that it remains dimensionally stable and holds the abrasive grits in place during subsequent "post-cure" processing. The backing with the abrasive grains held by a partially cured maker coat is then frequently provided with a size coat that is most often, but not essentially, chemically similar to the maker coat. This provides most of the grain retention function. This size coat must be dried and partially cured till it can be safely rolled without the overlying laps sticking to one another. The rolls are then conventionally placed in a controlled temperature environment to complete the cure. Because the rolls develop a temperature gradient from the outside to the core, care and time must be taken to ensure that this gradient is minimized to ensure uniformity of cure throughout the roll. This can take days to accomplish and implies that a huge amount of "work-in-progress" inventory is established. This is a source of great inefficiencies.
The problems in adequately and uniformly curing the binder resins derive in part from the nature of the cure process and the time needed to take the resin to the desired degree of cross-linking. In order to ensure the uniformity of the cure, the time is frequently measured in hours rather than minutes. Typically the drying and partial cure of the maker and size coats is carried out in ovens while draped over slats so as to hang in folds. These ovens, which are often referred to as "festoon ovens", have to be very large to accommodate the hanging folds and need to be in the form of temperature controlled zones held at temperatures appropriate to drying or curing operations. The curing zones are held at high temperatures for quite long periods to ensure controlled cure to the correct extent. Moreover such ovens are required not only to dry and partially cure the maker coat but also to dry and partially cure the size coat that is typically applied over it. Only after the correct level of cure of the top coating layer, (usually the size layer), has been obtained are the sheets rolled up and "post-cured" to the final desired level in that form. Drying and curing is therefore a very expensive operation in the production of coated abrasives.
Moreover the use of festoon ovens can lead to localized defects at the points where the sheets pass over the supporting slats. Stagnant air spots inside the oven between the folds can also lead to uneven heating of the sheets and uneven cure profiles across the sheets.
The present invention provides a novel way of heat-curing thermoset resins used in the manufacture of coated abrasives. This technique can readily be controlled, requires a fraction of the time to achieve the same degree of cure and potentially eliminates the need for festoon ovens altogether. This process is therefore capable of radically speeding up the production process and of significantly reducing the costs involved.